2014 May 12

Lecture for pulse monitor

Today’s class started with handing back the audio-amp lab reports, which spanned a much wider range than usual. Everyone had an ok design, but some students explained it well, while others had numbers that appeared by magic or with completely incoherent explanations. Although I could have predicted a couple of the worst reports, the group that did the best was not the one I expected. I’m quite pleased that they did such a good job, as I was not sure I was getting through to them—it made up for my disappointment at the relatively poor performance of a couple of the groups I had expected better of.

I pointed out to the students that there was no “correct” answer to most design problems, and that being able to explain how they came up with the design was at least as important as the design they came up with. If some spec changed, an engineer would want to be able to modify the design without having to do the whole thing over from scratch. (I didn’t say it, but I think that some of the groups that couldn’t explain their work would not be able to redo their designs from scratch, even if none of the specs changed.)

The multistep problems in last week’s lab and this week’s are difficult just because they have so many chained steps, though each individual step is pretty easy. I suspect that many of the students have never worked multistep problems before and are shutting down the moment they don’t have a predefined protocol to follow.

I asked how many people had done the prelab for tomorrow’s lab (as I had requested they do over the weekend). As expected, no one. I asked how many had attempted it—only about a third of the class. I asked where they were getting stuck—on the first step, figuring out how much light came out of the LED. Rather than going on and doing the rest of the chain of computations symbolically, they just gave up, so that they had nothing done, rather than an almost complete problem solved, with just one hole in it to be filled.

My lecture for most of the remaining hour consisted of explaining to them almost exactly the same thing that was in their lab handout. Luckily, I was expecting this inability or unwillingness of students to learn from written material, because I saw it last year also (though not on this lab, since this lab is all new). I would really love it if students would read things and at least try to do the assigned homework before class and come in with specific questions, rather than expecting to get everything in lectures for the first time. But I’ve resigned myself to students having less than zero initiative about learning new things.

Today had been scheduled for photodiodes and phototransistors, but we only got to those topics for the last 15 minutes of class, as I spent the first 55 minutes patiently going over what they needed to do to convert candelas to lumens to watts for the LEDs, to estimate how much light is absorbed or scattered in a finger, to estimate how much of the remaining light makes it to the sensor, and to compute how much current one would then see in the sensor. I didn’t do the computations for them (which seems to be what they expect—too much scaffolding in their other classes?), but I’m hoping that they can now read the homework assignment.

Despite my warnings that they would need to have the prelab done before lab starts tomorrow, or they are likely to run out of time for this lab, I’m betting that only one group will have gotten as far as a schematic, and that most of the class will again waste most of the lab time doing their prelab homework. I’ve not figured out a way to break them of this, but I’ll need to get better at getting students to work outside lab, or I won’t be able to handle two lab sections next year.

I am going to suggest that they write up half their lab report before Wednesday’s class, so that they can uncover the places where they can’t reconstruct their thinking before the final report is due on Friday, while there is still time to ask questions and modify the design.

On Wednesday, I plan to talk about the second stage of the amplifier for the pulse monitor, adding gain for 0.2Hz–30Hz but blocking DC. But figuring out how much gain they need requires them to have completed the first stage of the amplifier on Tuesday, and looking at the output with an AC-coupled oscilloscope, to see and measure the small fluctuation caused by opacity changes in the finger. I’m not sure that all groups will get that far, having not started on the design over the weekend as I told them to.

The class ended after a very brief and informal presentation of how a diode works, what causes photocurrent, and why the phototransistor has 100–1000× the current of a photodiode.

Sorry if I seem to be too much of a curmudgeon today—I’m very tired and even entirely expected behavior from the students was depressing. This isn’t even a “students-nowadays” complaint, as I remember having the same sort of disappointment about students being unwilling or unable to read assignments when I started as a professor 32 years ago. Perhaps there is some Shangri-La somewhere, where most students do the assigned reading and struggle to understand it before class rather than waiting to be spoonfed, but I’ve never taught there.

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[…] the design is any good is by following their chain of reasoning in creating it. As I mentioned in my previous post, some students are beginning to do a careful job of describing their design methods, while others […]

In your subsequent blog post you write: “I commit to staying in the lab until every group has completed a working project.” I wonder whether your students have any real consequences to not completing their pre-lab assignments if you give them unlimited lab time?

An interesting thought. But I’m not sure they have the foresight to avoid future bad consequences by doing work ahead of time. Many of them have conflicts (like other labs, classes, or dinner) after lab, and none have expressed a great desire to stay longer—they already see that as a negative outcome.

I had originally planned to collect the computations as homework the day before the lab, returning them on the day of the lab, but I couldn’t make the time for that quick a feedback cycle the first year so I gave up on it. Maybe I should reconsider it.